All Injectors share a common +12V supply voltage. The ECU injector output completes the circuit to ground when fuel delivery is required. When using the Haltech long flying lead harness this is already pre-terminated, injector power is supplied via a fused and relay switched circuit. If you are wiring your own harness you will need to ensure that injector power is fed into the ECU via the injector power pins. Failing to do this may result in unstable operation and electrical noise.